Rechargeable cells that use a nickel hydroxide positive electrode and a metal hydride forming hydrogen storage negative electrode ("metal hydride cells") are known in art.
When an electrical potential is applied between the electrolyte and a metal hydride electrode in a metal hydride cell, the negative electrode material (M) is charged by the electrochemical absorption of hydrogen and the electrochemical evolution of a hydroxyl ion; upon discharge, the stored hydrogen is released to form a water molecule and evolve an electron: ##STR1##
The reactions that take place at the positive electrode of a nickel metal hydride cell are also reversible. Most metal hydride cells use a nickel hydroxide positive electrode. The following charge and discharge reactions take place at a nickel hydroxide positive electrode: ##STR2## In a metal hydride cell having a nickel hydroxide positive electrode and a hydrogen storage negative electrode, the electrodes are typically separated by a non-woven, felled, nylon or polypropylene separator. The electrolyte is usually an alkaline aqueous electrolyte, for example, 20 to 45 weight percent potassium hydroxide.
The first hydrogen storage alloys to be investigated as battery electrode materials were TiNi and LaNi.sub.5. Many years were spent in studying these simple binary intermetallics because they were known to have the proper hydrogen bond strength for use in electrochemical applications. Despite extensive efforts, however, researchers found these intermetallics to be extremely unstable and of marginal electrochemical value due to a variety of deleterious effects such as slow discharge, oxidation, corrosion, poor kinetics, and poor catalysis. These simple alloys for battery applications reflects the traditional bias of battery developers toward the use of single element couples of crystalline materials such as NiCd, NaS, LiMS, ZnBr, NiFe, NiZn, and Pb-acid. In order to improve the electrochemical properties of the binary intermetallics while maintaining the hydrogen storage efficiency, early workers began modifying TiNi and LaNi.sub.5 systems.
The modification of TiNi and LaNi.sub.5 was initiated by Stanford R. Ovshinsky at Energy Conversion Devices (ECD) of Troy, Mich. Ovshinsky and his team at ECD found that reliance on simple, relatively pure compounds was a major shortcoming of the prior art. Prior work had determined that catalytic action depends on surface reactions at sites of irregularities in the crystal structure. Relatively pure compounds were found to have a relatively low density of hydrogen storage sites, and the type of sites available occurred accidently and were not designed into the bulk of the material. Thus, the efficiency of the storage of hydrogen and the subsequent release of hydrogen to form water was determined to be substantially less than that which would be possible if a greater number and variety of active sites were available.
Ovshinsky had previously found that the number of surface sites could be increased significantly by making an amorphous film that resembled the surface of the desired relatively pure materials. As Ovshinsky explained in Principles and Applications of Amorphicity, Structural Change, and Optical Information Encoding, 42 Journal De Physique at C4-1096 (Octobre 1981):
Amorphicity is a generic term referring to lack of X-ray diffraction evidence of long-range periodicity and is not a sufficient description of a material. To understand amorphous materials, there are several important factors to be considered: the type of chemical bonding, the number of bonds generated by the local order, that is its coordination, and the influence of the entire local environment, both chemical and geometrical, upon the resulting varied configurations. Amorphicity is not determined by random packing of atoms viewed as hard spheres nor is the amorphous solid merely a host with atoms imbedded at random. Amorphous materials should be viewed as being composed of an interactive matrix whose electronic configurations are generated by free energy forces and they can be specifically defined by the chemical nature and coordination of the constituent atoms. Utilizing multi-orbital elements and various preparation techniques, one can outwit the normal relaxations that reflect equilibrium conditions and, due to the three-dimensional freedom of the amorphous state, make entirely new types of amorphous materials--chemically modified materials. PA1 A disordered semiconductor can exist in several structural states. This structural factor constitutes a new variable with which the physical properties of the [material] . . . can be controlled. Furthermore, structural disorder opens up the possibility to prepare in a metastable state new compositions and mixtures that far exceed the limits of thermodynamic equilibrium. Hence, we note the following as a further distinguishing feature. In many disordered [materials] . . . it is possible to control the short-range order parameter and thereby achieve drastic changes in the physical properties of these materials, including forcing new coordination numbers for elements. . . . PA1 [S]hort-range order is not conserved . . . . Indeed, when crystalline symmetry is destroyed, it becomes impossible to retain the same short-range order. The reason for this is that the short-range order is controlled by the force fields of the electron orbitals therefore the environment must be fundamentally different in corresponding crystalline and amorphous solids. In other words, it is the interaction of the local chemical bonds with their surrounding environment which determines the electrical, chemical, and physical properties of the material, and these can never be the same in amorphous materials as they are in crystalline materials . . . The orbital relationships that can exist in three-dimensional space in amorphous but not crystalline materials are the basis for new geometries, many of which are inherently anti-crystalline in nature. Distortion of bonds and displacement of atoms can be an adequate reason to cause amorphicity in single component materials. But to sufficiently understand the amorphicity, one must understand the three-dimensional relationships inherent in the amorphous state, for it is they which generate internal topology incompatible with the translational symmetry of the crystalline lattice . . . . What is important in the amorphous state is the fact that one can make an infinity of materials that do not have any crystalline counterparts, and that even the ones that do are similar primarily in chemical composition. The spatial and energetic relationships of these atoms can be entirely different in the amorphous and crystalline forms, even though their chemical elements can be the same . . . Short-range, or local, order is elaborated on in U.S. Pat. No. 4,520,039 to Ovshinsky, entitled Compositionally Varied Materials and Method for Synthesizing the Materials, the contents of which are incorporated by reference. This patent discusses how disordered materials do not require any periodic local order and how, by using Ovshinsky's techniques, spatial and orientational placement of similar or dissimilar atoms or groups of atoms is possible with such increased precision and control of the local configurations that it is possible to produce qualitatively new phenomena. In addition, this patent discusses that the atoms used need not be restricted to "d band" or "f band" atoms, but can be any atom in which the controlled aspects of the interaction with the local environment plays a significant role physically, electrically, or chemically so as to affect the physical properties and hence the functions of the materials. These techniques result in means of synthesizing new materials which are disordered in several different senses simultaneously. PA1 (1) The combination of Mn and excess Fe may affect the bulk alloy by inhibiting the bulk diffusion rate of hydrogen within the metal through the formation of complex phase structures, either by effecting the grain boundaries or by affecting the equilibrium bond strength of hydrogen within the metal. In other words, the temperature dependance of the hydrogen bond strength may be increased thereby decreasing the available voltage and capacity available under low temperature discharge. PA1 (2) It is believed that the combination of Mn and excess Fe may result in a lower electrode surface area for metallurgical reasons by increasing the ductility of the alloy and thereby reducing the amount of surface area formation during the activation process. PA1 (3) It is believed that the combination of Mn and excess Fe to these alloys may inhibit low temperature discharge through the alteration of the oxide layer itself with respect to conductivity, porosity, thickness, and/or catalytic activity. The oxide layer is an important factor in the discharge reaction and promotes the reaction of hydrogen from the Base Alloy of the present invention and hydroxyl ion from the electrolyte. We believe this reaction is promoted by a thin, conductive, porous oxide having some catalytic activity.
Once amorphicity was understood as a means of introducing surface sites in a film, it was possible to produce "disorder" not only in amorphous materials, but also in crystalline materials; "disorder" that takes into account the entire spectrum of local order effects such as porosity, topology, crystallites, characteristics of sites, and distances between sites. Thus, rather than searching for material modifications that would yield ordered materials having a maximum number of accidently occurring surface irregularities, Ovshinsky's team at ECD began constructing "disordered" materials where the desired irregularities could be tailor made. See, U.S. Pat. No. 4,623,597, the disclosure of which is incorporated by reference.
The term "disordered," as used herein corresponds to the meaning of the term as used in the literature, such as the following:
S. R. Ovshinsky, The Shape of Disorder, 32 Journal of Non-Crystalline Solids at 22 (1979) (emphasis added).
The "short-range order" of these disordered materials are further explained by Ovshinsky in The Chemical Basis of Amorphicity: Structure and Function, 26:8-9 Rev. Roum. Phys. at 893-903 (1981):
By forming metal hydride alloys from such disordered materials, Ovshinsky and his team were able to greatly increase the reversible hydrogen storage characteristics required for efficient and economical battery applications, and produce batteries having high density energy storage, efficient reversibility, high electrical efficiency, bulk hydrogen storage without structural change or poisoning, long cycle life, and deep discharge capability.
The improved characteristics of these alloys result from tailoring the local chemical order and hence the local structural order by the incorporation of selected modifier elements into a host matrix. Disordered metal hydride alloys have a substantially increased density of catalytically active sites and storage sites compared to simple, ordered crystalline materials. These additional sites are responsible for improved efficiency of electrochemical charging/discharging and an increase in electrical energy storage capacity. The nature and number of storage sites can even be designed independently of the catalytically active sites. More specifically, these alloys are tailored to allow bulk storage of the dissociated hydrogen atoms at bonding strengths within the range of reversibility suitable for use in secondary battery applications.
Based on the pioneering principles described above, some of the most efficient electrochemical hydrogen storage materials were formulated. These are the Ti--V--Zr--Ni type active materials such as disclosed in U.S. Pat. No. 4,551,400 ("the '400 Patent") to Sapru, Hong, Fetcenko, and Venkatesan, the disclosure of which are incorporated by reference. These materials reversibly form hydrides in order to store hydrogen. All the materials used in the '400 Patent utilize a generic Ti--V--Ni composition, where at least Ti, V, and Ni are present with at least one or more of Cr, Zr, and Al. The materials of the '400 Patent are multiphase materials, which may contain, but are not limited to, one or more AB.sub.2 phases with C.sub.14 and C.sub.15 type crystal structures. The following formulae are specifically disclosed in the '400 Patent: EQU (TiV.sub.2-x Ni.sub.x).sub.1-y M.sub.y
where x is between 0.2 and 1.0; y is between 0.0 and 0.2; and M=Al or Zr; EQU Ti.sub.2-x Zr.sub.x V.sub.4-y Ni.sub.y
where Zr is partially substituted for Ti; x is between 0.0 and 1.5; and y is between 0.6 and 3.5; and EQU Ti.sub.1-x Cr.sub.x V.sub.2-y Ni.sub.y
where Cr is partially substituted for Ti; x is between 0.0 and 0.75; and y is between 0.2 and 1.0.
Other Ti--V--Zr--Ni alloys may also be used for a rechargeable hydrogen storage negative electrode. One such family of materials are those described in U.S. Pat. No. 4,728,586 ("the '586 Patent") to Venkatesan, Reichman, and Fetcenko for Enhanced Charge Retention Electrochemical Hydrogen Storage Alloys and an Enhanced Charge Retention Electrochemical Cell, the disclosure of which is incorporated by reference. The '586 Patent describes a specific sub-class of these Ti--V--Ni--Zr alloys comprising Ti, V, Zr, Ni, and a fifth component, Cr.
In a particularly preferred exemplification of the '586 Patent, the alloy has the composition EQU (Ti.sub.2-x Zr.sub.x V.sub.4-y Ni.sub.y).sub.1-z Cr.sub.z
where x is from 0.00 to 1.5, y is from 0.6 to 3.5, and z is an effective amount less than 0.20. These alloys may be viewed stoichiometrically as comprising 80 atomic percent of a V--Ti--Zr--Ni moiety and up to 20 atomic percent Cr, where the ratio of (Ti+Zr+Cr+optional modifiers) to (Ni+V+optional modifiers) is between 0.40 to 0.67. The '586 patent, mentions the possibility of additives and modifiers beyond the Ti, V, Zr, Ni, and Cr components of the alloys, and generally discusses specific additives and modifiers, the amounts and interactions of these modifiers, and the particular benefits that could be expected from them.
The V--Ti--Zr--Ni family of alloys described in the '586 Patent has an inherently higher discharge rate capability than previously described alloys. This is the result of substantially higher surface areas at the metal/electrolyte interface for electrodes made from the V--Ti--Zr--Ni materials. The surface roughness factor (total surface area divided by geometric surface area) of V--Ti--Zr--Ni alloys is about 10,000. This value indicates a very high surface area and is supported by the inherently high rate capability of these materials.
The characteristic surface roughness of the metal/electrolyte interface is a result of the disordered nature of the material. Since all of the constituent elements, as well as many alloys and phases of them, are present throughout the metal, they are also represented at the surfaces and at cracks which form in the metal/electrolyte interface. Thus, the characteristic surface roughness is descriptive of the interaction of the physical and chemical properties of the host metals as well as of the alloys and crystallographic phases of the alloys, in an alkaline environment. These microscopic chemical, physical, and crystallographic parameters of the individual phases within the hydrogen storage alloy material are believed to be important in determining its macroscopic electrochemical characteristics.
In addition to the physical nature of its roughened surface, it has been observed that V--Ti--Zr--Ni alloys tend to reach a steady state surface composition and particle size. This steady state surface composition is characterized by a relatively high concentration of metallic nickel. These observations are consistent with a relatively high rate of removal through precipitation of the oxides of titanium and zirconium from the surface and a much lower rate of nickel solubilization, providing a degree of porosity to the surface. The resultant surface seems to have a higher concentration of nickel than would be expected from the bulk composition of the negative hydrogen storage electrode. Nickel in the metallic state is electrically conductive and catalytic, imparting these properties to the surface. As a result, the surface of the negative hydrogen storage electrode is more catalytic and conductive than if the surface contained a higher concentration of insulating oxides.
The surface of the negative electrode, which has a conductive and catalytic components, such as the metallic nickel, catalyzes various charge and discharge reaction steps.
In contrast to the Ti--V--Zr--Ni based alloys described above, alloys of the AB.sub.5 type have generally been considered "ordered" materials that have a different chemistry and microstructure, and exhibit different electrochemical characteristics compared to the Ti--V--Zr--Ni alloys. However, analysis reveals while the early AB.sub.5 alloys may have been ordered materials, more recently developed AB.sub.5 alloys are not. The performance of the early ordered AB.sub.5 materials was poor. However, presently used AB.sub.5 alloys have a high degree of modification (that is as the number and amount of elemental modifiers increased) and the performance of the AB.sub.5 alloys has improved significantly. This is due to the disorder contributed by the modifiers as well as their electrical and chemical properties. This evolution of AB.sub.5 type alloys from a specific class of "ordered" materials to the current multicomponent, multiphase "disordered" alloys that are now very similar to Ti--V--Zr--Ni alloys is shown in the following patents: (i) U.S. Pat. No. 3,874,928; (ii) U.S. Pat. No. 4,214,043; (iii) U.S. Pat. No. 4,107,395; (iv) U.S. Pat. No. 4,107,405; (v) U.S. Pat. No. 4,112,199; (vi) U.S. Pat. No. 4,125,688; (vii) U.S. Pat. No. 4,214,043; (viii) U.S. Pat. No. 4,216,274; (ix) U.S. Pat. No. 4,487,817; (x) U.S. Pat. No. 4,605,603; (xii) U.S. Pat. No. 4,696,873; and (xiii) U.S. Pat. No. 4,699,856. (These references are discussed extensively in U.S. Pat. No. 5,096,667 and this discussion is specifically incorporated by reference.)
Simply stated, in the AB.sub.5 alloys, like the Ti--V--Zr--Ni alloys, as the degree of modification increases, the role of the initially ordered base alloy becomes of secondary importance compared to the properties and disorder attributable to the particular modifiers. In addition, analysis of the current multiple component AB.sub.5 alloys indicates that current AB.sub.5 alloy systems are modified following the guidelines established for AB.sub.2 systems. Thus, highly modified AB.sub.5 alloys are identical to AB.sub.2 alloys in that both are disordered materials that are characterized by multiple-components and multiple phases and there no longer exists any significant distinction between these two types of multicomponent, multiphase alloys.